Stop measuring dirauths

Working on sbws#40049 (closed) i've found that the dirauths are being measured even if the variable in the configuration is off. Since torflow is not measuring them, sbws should stop measuring them too. I think it's due to the lines:

if not self.measure_authorities:
    relays = relays.difference(set(self.relay_list.authorities))

Probably cause the object id is different in the 2 lists.